As with all other tires, motorbike tires too are going radial, the architecture of such tires comprising a carcass reinforcement made up of one or two layers of reinforcing elements which with the circumferential direction make an angle that may be between 65 and 90°, the said carcass reinforcement being radially surmounted by a crown reinforcement made up of reinforcing elements. However, some non-radial tires do still remain and the invention relates to these also. The invention further relates to tires which are partially radial, which means tires in which the reinforcing elements of the carcass reinforcement are radial over at least part of the said carcass reinforcement, for example in the part corresponding to the crown of the tire.
Numerous crown reinforcement architectures have been proposed, depending on whether the tire is intended to be fitted at the front of the motorbike or fitted at the rear. A first structure, for the said crown reinforcement, involves using only circumferential cords, and the said structure is more particularly used for rear tires. A second structure, inspired directly by the structures commonly used on passenger vehicle tires, has been used to improve resistance to wear, and involves using at least two working crown layers of reinforcing elements that are substantially parallel to one another within each layer but crossed from one layer to the next, making acute angles with the circumferential direction, such tires being more particularly suitable as front tires for motorbikes. The said two working crown layers may be combined with at least one layer of circumferential elements, generally obtained by helical winding of a strip of at least one rubber-coated reinforcing element.
The choice of tire crown architecture has a direct impact on certain properties of the tire, such a wear, endurance, grip or even drivability or, particularly in the case of motorbikes, stability. However, other tire parameters such as the nature of the rubber compounds of which the tread is made also have an impact on the properties of the said tire. The choice and nature of the rubber compounds of which the tread is made are, for example, essential parameters as far as wear properties are concerned. The choice and nature of the rubber compounds of which the tread is made also have an impact on the grip of the tire.
It is also known practice for other types of tire to produce treads comprising incisions, more particularly for tires intended to run on ground covered with snow, black ice, or wetness.
Such treads are usually provided with raised elements of the rib or block type, separated from one another in the circumferential direction and/or in the transverse direction by transverse and/or circumferential grooves. These treads then also comprise incisions or slits, the non-zero widths of which are very much smaller than those of the aforementioned grooves. By making a plurality of cuts that open onto the tread surface a plurality of rubber edges is created and these cut into the layer of water that may be present on the road surface in order to keep the tire in contact with the ground and create cavities that may potentially form ducts intended to collect the water present in the contact patch via which the tire makes contact with the road and remove if it they are configured in such a way as to open out outside the contact patch.
Numerous types of incision have already been proposed with a view to improving the grip of the tire on the surfaces in question.
Document FR 2 418 719 for example describes incisions which may be normal to the surface of the tread or inclined with respect to the direction perpendicular to the said surface.
Document FR 791 250 describes incisions that run in a wave along the surface of the tread.
Motorcycle performance means that nowadays better control over the transfer of torque is desired, this notably meaning driving torque in terms of the rear wheel and braking torque in terms of the front wheel.
As stated previously, the architecture of the crown reinforcement of the tire or indeed the nature of the rubber compounds of the tread may allow such effects to be obtained.
Torque transfer is also dependent on the area of contact with the ground. Now, curvatures that are particularly pronounced, notably in the axial direction, in motorcycle tires do nothing to encourage the flattening of the tire when riding in a straight line. The tread is loaded in compression and this effectively limits the surface area of the contact patch by which the tire is in contact with the ground and, what is more, leads to energy losses.
Circumferential furrows have already been proposed in an attempt to limit this compressive loading of the tread. The result is satisfactory from an energy standpoint but the lack of rubber compound corresponding to the location of the furrows decreases the area of tire in contact with the ground in the contact patch and therefore limits the force that can be transmitted under braking, particularly in the case of the front tire or during acceleration in the case of the rear tire.